Recently, there is a tendency to impose strict requirements on the discharge of organic solvents, in order to prevent pollution of the environments to be caused by the evaporation of the organic solvents used in coating compositions. Accordingly, there have been widely investigated aqueous coating compositions without using or without substantially using organic solvents, powder coating compositions and ultraviolet ray curable coating compositions. However, those coating compositions do not provide any sufficient coating properties. For example, the aqueous coating compositions are poor in water resistance and still need at least about 20% of organic solvents. The powder coating compositions need resins having a very higher glass transition temperature. Therefore, there is a problem in that the coating obtained from the powder coating compositions is brittle and cannot meet the requirements of scuff resistance as required for overlayer coatings and interlayer coatings.
On the other hand, there have also been investigated organic solvent type-coating compositions for further reducing the content of the organic solvents. For example, Japanese Patent Unexamined Publication (hereinafter referred to as "Kokai") No. 58-108259 discloses a high solid content coating composition comprising alkoxy melamine resin, a polyol or oligomer having a specific hydoxyl value and an acid catayst, at a specific proportion, so as to make the molecular weight smaller, whereby increasing the solid resin content when the coating composition is coated, while reducing the amount of the organic solvent to be used in the coating composition.
However, in order to supplement the insufficiency of curing which is caused by the lowered molecular weight of the oligomer, the amount of a hydroxyl group must be increased. Since the hydroxyl group has a strong polarity, it is difficult to reduce the viscosity. In addition, when resin having a high hydroxyl value is cured with melamine resin, the performances of the resultant coating, such as acid resistance, etc. were much reduced.